In Vivo Effects of Zoledronic Acid on Bone Remodeling.

Zoledronic acid (ZA) is an amino-bisphosphonate with very potent antiresorptive activity widely used in the treatment of multiple myeloma (MM) bone disease. Recently, the increasing incidence of osteonecrosis of the jaw and its possible association with prolonged use of amino-bisphosphonates such as ZA has been reported. Therefore, we here studied the effects of ZA on bone remodeling in vitro and in vivo. Although ZA has been shown to inhibit osteoclastogenesis and increase bone mineralization, data on its effects on osteoblastic activity and on bone formation are conflicting. To study the effects of ZA on osteoblasts (OB), 5 week old C57BL/6 mice were treated with low and high doses of ZA intraperitoneally (IP) weekly. The dose ranged from 0.05-1mg/kg, with the highest dose recapitulating a lifetime dose of ZA over a 5 year period in an adult MM patient. Blood was collected at baseline and weekly thereafter. IP calcein injections were administered to study bone formation rates. Consistent with its known effects on bone mass and density, in vivo DXA scans in ZA-treated animals demonstrated an increase in whole body bone mineral density (BMD). ZA treatment was associated with a dose-related increase in trabecular bone at the distal femur, evaluated by microCT and confirmed by static histomorphometry. ELISA assays showed a decrease in TRACP5B (bone resorption), as expected due to the anti-osteoclastic activity of ZA. In addition to osteoclast (OC) inhibition, mice treated with ZA also showed alterations in OB activity. Specifically, serum osteocalcin (a marker for bone formation) levels were lower in ZA treated mice, and dynamic histomorphometry confirmed decreased bone formation rates. In order to study the mechanism of our in vivo observations, we tested the effects of ZA on OB, OC, and bone marrow stromal cells (BMSCs) treated in vitro with ZA (0.01μM to 100μM) for 7, 14, and 21 days. Decreased viability during differentiation of both OB and OC was observed, without any significant effects on BMSCs. This was associated with decreased alkaline phosphatase activity, suggesting functional impairment of OB activity. Our data therefore suggests that ZA impairs OB number and activity, in addition to effects on OC and may impair normal physiologic bone remodeling. Ongoing studies will determine the molecular mechanisms whereby ZA mediates these sequelae and inform future studies of ZA use in patients with MM bone disease.